Products formed by joining two or more thin sheets of thermoplastic materials have many commercial and industrial applications. For example, jackets or pockets are produced by feeding sheets of thermoplastic materials to a seaming machine where the sheets are joined along welded seams formed at their edges. Covers for ring binders that surround a stiff insert are produced in a similar manner. The stiff insert, for example a rigid piece of cardboard, is positioned between the thermoplastic sheets before the welded seams are formed along the edges of the ring binder cover. Welded seams may also be formed along predetermined lines extending between opposed edges of the sheets to create folds or "hinges". Medical products such as blood bags, catheters and compression sleeves produced from thin sheets of vinyl, polyvinyl chloride (PVC) or polyurethane in a similar manner may also utilize welded seams. In particular, these medical products may include one or more gas or fluid retaining bladders created by selectively forming interior welded seams on the thermoplastic sheets.
Welding machines used to form such jackets, pockets, ring binder covers and medical products employ welding heads which typically include cutting edges that produce the contour desired for the particular item. As will be discussed hereinafter, the welding machine may also employ a heating assembly which is in thermal communication with each welding head. The heating assembly is used to soften, or plasticize, the thermoplastic sheets in the area of the welded seam under applied pressure so that the sheets fuse together when the heat and pressure are removed. The area of the weld is generally preheated to between about 140.degree. F. to about 160.degree. F. so that an integral weld is achieved when the plasticized sheets fuse together. At the same time that the welded seams are formed, the cutting edges of the welding machine create the contour desired for the particular item.
A number of different methods have been proposed for welding thermoplastic materials. For example, thermocontact welding, also known as "hot die welding" or "thermo welding" involves applying pressure to thin sheets of thermoplastic materials positioned between a heated top platen having a top die, and a heated bottom platen having a bottom die. As illustrated in FIG. 1a, a pair of opposed thermoplastic sheets 18 are positioned between the top die 12 of the top platen 10 and the bottom die 16 of the bottom platen 14. Assuming that the top and bottom dies are precisely matched, a welded seam is formed when heat is transferred from the top and bottom platens 10, 14 through the top and bottom dies 12, 16 to the area of the weld and plasticizes the thermoplastic sheets 18 while, at the same time, the top and bottom dies apply pressure to the sheets along the welded seam.
An unfortunate consequence of the hot die welding process is that the heat lost during thermal transfer from the platens creates a temperature gradient in the thermoplastic sheets. As a result, the temperature of the thermoplastic sheets 18 adjacent the top and bottom dies 12, 16 is greater than the temperature away from the dies, and particularly, is greater than the temperature in the area of the welded seam. To compensate for this temperature gradient, the temperature of the heated platens 10, 14 must be maintained well above the temperature at which the particular thermoplastic materials plasticize. As used herein, the temperature at which the materials of the sheets plasticize is termed the "softening temperature." The temperature across the entire thickness of the thermoplastic sheets 18, and most importantly in the area of the welded seam, is thus maintained above the softening temperature of the particular material of the thermoplastic sheets. Alternatively, the thermoplastic sheets 18 may be pressed together for an extended period of time to permit the heat transferred from the heated platens 10, 14 to accumulate, and thus cause the temperature in the area of the welded seam to rise above the softening temperature of the particular material of the thermoplastic sheets.
The dies 12, 16 used in thermocontact welding are generally made of materials that exhibit good heat conductivity and heat retention to provide relatively quick heat transfer from the heated platens 10, 14, and thus a relatively fast welding process. As a result thermocontact welding provides essentially no cooling time under applied pressure prior to release once the thermoplastic sheets plasticize. In addition, the outer surfaces of the thermoplastic sheets 18 in contact with the hot dies 12, 16 typically plasticize (even in the presence of a heat-resistant coating) before the inner surfaces of the thermoplastic sheets in the area of the welded seam. As should be apparent, the potential therefore exists with hot die welding that the integrity of the welded seam will be poor and that the thermoplastic sheets will adhere to the hot die surfaces. Thus, it is necessary to take additional measures to insure that an integral welded seam is achieved and that the thermoplastic sheets 18 will release from the top and bottom dies 12, 16 when the top and bottom platens 10, 14 are separated.
Alternatively, the area of the weld may be plasticized by subjecting the thermoplastic sheets to high frequency radiation, such as in ultrasonic welding, induction welding or radio frequency (RF) welding processes. RF welding is also known as RF heat sealing, high frequency sealing, and dielectric heat sealing. For example, FIG. 1b illustrates an RF welding apparatus which employs an RF energy generator 20 that is electrically connected between a top platen 22 and a bottom platen 24. The top platen 22 has a buffer 26, such as a dielectric insulating material, attached to the inner surface of the top platen. The bottom platen 24 has an upwardly facing welding die 28 attached to the inner surface of the bottom platen which may be hot, but preferably is cold. A pair of opposed thermoplastic sheets 18 made of vinyl, for example, are positioned between the buffer material 26 of the top platen 22 and the welding die 28 of the bottom platen 24.
A force is applied to the top platen 22 and/or the bottom platen 24 so that the thermoplastic sheets 18 are pressed together. RF energy from the energy generator 20 is then applied to the thermoplastic sheets 18 through the bottom platen 24 and the welding die 28. In particular, the RF energy is directed into the area of the weld so that the molecules of the thermoplastic materials oscillate at high frequency, thereby generating localized heat. The combination of the heat generated by the RF energy in the area of the welded seam and the pressure exerted on the thermoplastic sheets 18 by the welding die 28 causes the sheets to plasticize along the welded seam and to fuse together once the RF energy is removed. The ability and ease with which the thermoplastic sheets 18 plasticize and fuse together is related to the dielectric properties of the particular material of the thermoplastic sheets.
Because heat is generated at the molecular level throughout the thickness of the thermoplastic sheets 18, RF welding plasticizes the area of the weld more rapidly and evenly than thermocontact welding. High frequency energy, however, passes through thermoplastic materials, typically monomers, that exhibit a low absorption of RF energy. Thus, RF welding is most effective when used with di-polar thermoplastic materials exhibiting a high enough absorption of RF energy to generate sufficient heat in the area of the welded seam to produce an integral welded seam. Such materials are also referred to herein as "RF receptive." Consequently, RF welding techniques are not suitable for welding thermoplastic sheet materials that do not readily absorb RF energy (i.e., are not RF receptive).
RF welding techniques are occasionally used to weld thin polyurethane film. However, a high intensity of RF energy is required to generate sufficient heat in the area of the welded seam to guarantee the integrity of the welded seam. Polyurethane film ranging in thickness from about 0.5 to about 1.5 mils (0.0127 to 0.0381 mm) exhibits a very low absorption of RF energy because of the microcrystalline structure of the film. In thin film applications of RF welding, the RF energy required to achieve seam integrity typically causes sparking due to arcing which can result in burns and chars to the film as well as to the welding surfaces of the top and bottom platens 22, 24. Arcing can also be hazardous to the operator of the RF welding machine. For these reasons, RF welding is generally not employed for polyurethane film less than about 3 mils (0.0762 mm) in thickness.
Vinyl and PVC are particularly suitable for RF welding because of their affinity to absorb RF energy. The molecules within vinyl and PVC materials are responsive to periodic stresses caused by an RF energy field alternating in polarity only a relatively few million times per second, such as at 27.12 Mhz. The amount of heat developed in the materials is directly proportional to the amount of RF energy applied and absorbed. As a result, the most common application of RF welding is PVC bonding of thin sheets to produce medical products having gas or fluid retaining bladders created by selectively forming interior welded seams on the PVC sheets.
However, vinyl and PVC are difficult materials to dispose of without negatively impacting the environment. Medical products incorporating vinyl or PVC cannot be incinerated without releasing toxins. Accordingly, additional measures must be employed to capture the toxins discharged when vinyl or PVC materials are burned. On the other hand, polyolefins, such as polypropylene or polyethylene materials, do not release toxins when incinerated. Accordingly, polyolefin materials are preferred by the medical plastics industry for their limited environmental impact and, thus relative ease of disposal. Because polypropylene is substantially chemically inert and is relatively resistant to extreme temperatures (i.e., is not susceptible to cold-cracking and thermal aging), it is a particularly suitable material for incorporation in medical products. Unfortunately, unlike vinyl and PVC, thin sheets of polyolefin based materials cannot be readily joined by RF welding because they exhibit a relatively low absorption of RF energy.
Accordingly, thin sheets of polyolefin based materials are typically joined by thermocontact welding even though attempts to produce an integral welded seam by thermocontact welding have previously caused the thin thermoplastic sheets to exhibit a tendency to curl or otherwise deform. In an attempt to prevent curling and deformation of welded polyolefin sheets, thermocontact methods for welding polypropylene sheets have utilized specially designed hot die surfaces which produce discontinuous or intermittent welded seams (i.e., the welded seam consists of a series of short welds with non-welded segments interspersed between successive pairs of the short welds). Thermoplastic products having intermittent welded seams, however, have only limited applications.
For example, medical products that include internal bladders to retain fluids, such as blood bags and sequential compression devices, cannot utilize intermittent welded seams. Similarly, medical products that are exposed to bodily fluids cannot utilize intermittent welded seams because of the accompanying risk of disease transmission. Additionally, only thin (i.e., less than about 4.0 mils) sheets of polyethylene film are responsive to intermittent welding techniques, thereby further limiting the applicability of polyolefin-based thin sheet thermoplastic materials for the manufacture of medical products.
Thin sheets of dissimilar thermoplastic materials may be joined by impulse welding techniques. As illustrated in FIG. 1c, thin sheets of dissimilar thermoplastic sheets 18a, 18b are positioned between a top platen 30 and a heated support platen 32. The top platen 30 includes a primary heat barrier 34 attached to the inner surface of the top platen and a secondary heat barrier 36 positioned adjacent the thermoplastic sheets 18a, 18b. The heat barriers 34, 36 may be made of heat insulating material such as silicone rubber. A resistance wire 38 is positioned between the primary heat barrier 34 and the secondary heat barrier 36. The resistance wire 38 is made of a material that can be heated and cooled rapidly, such as nichrome.
Typically, the sheet of the dissimilar thermoplastic materials 18a, 18b having the higher melt point is positioned nearest the resistance wire 38. To effectively join the dissimilar thermoplastic materials 18a, 18b, the resistance wire 38 is first rapidly heated to a temperature above the softening temperature of the thermoplastic sheet having the higher melt point to plasticize both sheets. The resistance wire 38 is then rapidly cooled while the support platen 32 is conventionally heated to a temperature lower than the softening temperature of the other thermoplastic sheet to permit the sheets to fuse together. The rapid heating and cooling of the resistance wire 38 is necessary to initiate the melt cycle of the thermoplastic material having the higher melt point, thereby permitting molecular bonding of the dissimilar materials and congealing of the resulting welded seam. If the heating and cooling of the resistance wire 38 are not accurately controlled, the welded seam may have undesirable weak or thinning characteristics.
A disadvantage of the common welding techniques is that they each require that the thermoplastic sheets be subjected to an prolonged residence time in the welding machine. In thermocontact welding, once the temperature of the thermoplastic sheets is above the softening temperature of the materials, the hot dies apply pressure to the thermoplastic sheets for a period of time to form the welded seam. In impulse welding, once the heat cycle of the resistance wire 38 is completed, the secondary heat barrier and the support platen apply pressure to the thermoplastic sheets for a period of time to form the welded seam. In RF welding, the buffer material and the welding die likewise apply pressure to the thermoplastic sheets for a period of time (although typically for less time than the residence time required in thermocontact and impulse welding techniques) so that the RF energy can generate sufficient heat to form the welded seam. Naturally, it is desirable to minimize the residence time of the thermoplastic sheets in the welding machine to insure that the heated surfaces release from the thermoplastic sheets (as previously described), as well as to increase the production rate of the thermoplastic sheet products.
Therefore, it is apparent that a need exists for an improved apparatus and method for welding thermoplastic materials. More particularly, a need exists for an apparatus and method for joining two or more thin sheets of thermoplastic materials along a welded seam. Further, a need exists for an apparatus and method for joining two or more thin thermoplastic sheets along a welded seam having excellent seam integrity. Still further, a need exists for an apparatus and method for rapidly welding thermoplastic sheets to insure that the heated surfaces of the welding apparatus release from the thermoplastic sheets as well as to increase the production rate of products made from thermoplastic sheet materials.